MAX4178ESA-T Maxim Integrated, MAX4178ESA-T Datasheet - Page 8

• Manufacturer Part Number: MAX4178ESA-T
• Description: Precision Amplifiers
• Manufacturer: Maxim Integrated

330MHz, Gain of +1/Gain of +2
Closed-Loop Buffers
In order to obtain the MAX4178/MAX4278s’ full 330MHz/
310MHz bandwidths, microstrip and stripline tech-
niques are recommended in most cases. To ensure
that the PC board does not degrade the amplifier’s per-
formance, it’s a good idea to design the board for a fre-
quency greater than 1GHz. Even with very short traces,
it’s good practice to use these techniques at critical
points, such as inputs and outputs. Whether you use a
constant-impedance board or not, observe the follow-
ing guidelines when designing the board:
On Maxim’s evaluation kit, the ground plane has been
removed from areas where keeping the trace capaci-
tance to a minimum is more important than maintaining
ground continuity.
The MAX4178/MAX4278 provide maximum AC perfor-
mance with no output load capacitance. This is the
case when the MAX4178/MAX4278 are driving a cor-
rectly terminated transmission line (e.g., a back-termi-
nated 75
are capable of driving capacitive loads up to 100pF
without oscillations, but with reduced AC performance.
Driving large capacitive loads increases the chance of
oscillations in most amplifier circuits. This is especially
true for circuits with high loop gains, such as voltage
followers. The amplifier’s output resistance and the load
8
Do not use wire-wrap boards. They are too inductive.
Do not use IC sockets. They increase parasitic capaci-
tance and inductance.
In general, surface-mount components have shorter
leads and lower parasitic reactance, giving better
high-frequency performance than through-hole com-
ponents.
The PC board should have at least two layers, with
one side a signal layer and the other a ground plane.
Keep signal lines as short and straight as possible.
Do not make 90° turns; round all corners.
The ground plane should be as free from voids as
possible.
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cable). However, the MAX4178/MAX4278
Applications Information
Driving Capacitive Loads
Grounding, Bypassing,
and PC Board Layout
capacitor combine to add a pole and excess phase to
the loop response. If the frequency of this pole is low
enough and if phase margin is degraded sufficiently,
oscillations may occur.
A second problem when driving capacitive loads
results from the amplifier’s output impedance, which
looks inductive at high frequency. This inductance
forms an L-C resonant circuit with the capacitive load,
which causes peaking in the frequency response and
degrades the amplifier’s gain margin.
The MAX4178/MAX4278 drive capacitive loads up to
100pF without oscillation. However, some peaking (in
the frequency domain) or ringing (in the time domain)
may occur. This is shown in Figures 2a and 2b and the
in the Small- and Large-Signal Pulse Response graphs
in the Typical Operating Characteristics.
To drive larger-capacitance loads or to reduce ringing,
add an isolation resistor between the amplifier’s output
and the load, as shown in Figure 1.
The value of R
capacitive load. Figures 3a and 3b show the Bode
plots that result when a 20
with a voltage follower driving a range of capacitive
loads. At the higher capacitor values, the bandwidth is
dominated by the RC network, formed by R
the bandwidth of the amplifier itself is much higher.
Note that adding an isolation resistor degrades gain
accuracy. The load and isolation resistor form a divider
that decreases the voltage delivered to the load.
Figure 1. Capacitive-Load Driving Circuit
V
IN
MAX4178
MAX4278
ISO
depends on the circuit’s gain and the
R
ISO
isolation resistor is used
C
L
ISO
R
and C
L
V
OUT
L
;